In London, with few exceptions, the cellular connectivity is better than whatever horrendous WiFi your hotel/restaurant/train/etc. has. The only time WiFi is sometimes better is in cool underground bars and such where there’s no cellular signal whatsoever.

Hell it’s better than most people’s home internet. My home internet is 1.1Gb/s at the wall, but once you factor in the WiFi in a dense urban environment with a brick wall house, I’m lucky if I get 150Mb/s. Meanwhile 5G from my bedroom easily clears 500Mb/s.

WiFi isn't available everywhere. Even when it is, it can be pretty unreliable or require you to enter information into sketchy captive portals. In general, for a mobile computer, mobile data is quite useful.

If you only use your laptop at home, then it's not terribly useful. But then at that point you might be better off with a desktop computer.